What are type annotations in Python?

Type annotations are a way to declare the expected types for function arguments, return values, and variables. They do not affect the runtime behavior of the program but serve as helpful aids during development.

Basic syntax:

def greet(name: str) -> str:
    return f"Hello, {name}!"

age: int = 25
price: float = 19.99
is_active: bool = True

Collections:

# Python 3.9+
def process(items: list[str]) -> dict[str, int]:
    return {item: len(item) for item in items}

# Nested types
matrix: list[list[int]] = [[1, 2], [3, 4]]

The typing module:

from typing import Optional, Union

# Can be str or None
def find_user(user_id: int) -> Optional[str]:
    if user_id == 1:
        return "Anna"
    return None

# Can be int or str
def parse(value: Union[int, str]) -> str:
    return str(value)

Why use annotations:

• Documentation: the expected types are clear without reading the function body.
• IDEs: autocomplete and code suggestions work better.
• Static analysis: tools like mypy can catch type errors prior to running code.
• Collaboration: makes the codebase easier for other developers to understand.

Important note:

Annotations are merely hints, not strict enforcements. Python does not enforce types at runtime:

def add(a: int, b: int) -> int:
    return a + b

add("hello", " world")  # Will run without error yielding: "hello world"